Several minor events/notifications can happen based on your pet's growth. These occur the morning after the day your pet reaches the required growth level.

At 50% growth, he goes from a single tentacle to multiple in an anemone looking form. This also changes what happens when petting him.
At 100% growth, he turns into a fleshy egg sac (cocoon) while he undergoes his metamorphosis. Petting changes again, and he can not be fed.
After a little over 2 weeks, he will hatch into his adult form, which I made to be super cute for a loveable tentacle pet.
If you feed him the maximum amount each day, it will take 20 days for him to start his metamorphosis.
Adult form tentacle pet is not complete at all, however you can still visit him to pet him. He'll be a lot more involved than this when actually finished.